Sperm adhesion in sea urchins is mediated by bindin, a protein that coats the acrosomal process at the apex of the sperm head. The long term objectives of this project are to provide an understanding of the structure of bindin and its function in fertilization. Bindin specifically associates with gel-phase phospholipid vesicles and can induce the fusion of vesicles in vitro. We propose to examine whether these properties of bindin are important of maintaining the association of bindin with the acrosomal process and for catalyzing the fusion of the sperm and egg plasma membranes in vivo. We will investigate the manner in which the bindin polypeptide associates with the phospholipid bilayer and how this association is regulated by the gel/fluid state of the bilayer. The ability of bindin to specifically associate with gel-phase bilayers may be useful for studying gel-phase domains in biological membranes and suggests a novel regulatory mechanism whereby the physical state of the membrane can modulate the function of the membrane by controlling the proteins associated with the membrane. We also propose to investigate whether the ability of bindin to induce the fusion of vesicles in vitro is related to the fusagenic properties of the acrosomal process in vivo. It is well established the electrical potential across the egg plasma membrane regulates the ability of the sperm to fuse with the egg membrane. In this context it is interesting to note that bindin shares considerable sequence homology with a region of the sodium channel (including a unique segment of 8 contiguous glutamic residues) which has been suggested to be a voltage sensing element. We plan to examine the effect of altering the electrical potential across the vesicle membrane on the ability of bindin to induce the fusion of phospholipid vesicles with cellular plasma membranes. This could be potentially very useful for delivery of drugs and macromolecules such as nucleic acids into the cytoplasm of cells. We propose to investigate the molecular basis for the specific interaction of sulfated, fucoe-containing glycoconjugates with bindin determine whether these glycoconjugates serve as species-specific egg surface receptors for bindin during sperm adhesion. These investigations should advance our understanding of the molecular basis of gamete interaction which could be useful for understanding the causes of infertility and serve as the basis for the development of novel contraceptives.